Stereoselective 1,3-dipolar cycloadditions of a chiral nitrone derived from erythrulose. An experimental and DFT theoretical study

We have investigated several 1,3-dipolar cycloadditions of a chiral nitrone prepared from L-erythrulose. While cycloadditions to carbon-carbon multiple bonds of dipolarophiles such as ethyl acrylate, ethyl propiolate, or dimethyl acetylenedicarboxylate were poorly stereoselective, reaction with acrylonitrile provided predominantly one diastereomeric adduct. Furthermore, the regioselectivity exhibited by the two structurally similar dipolarophiles ethyl acrylate and ethyl propiolate was found to be opposite. The molecular mechanisms of these cycloadditions have thus been investigated by means of density functional theory (DFT) methods with the B3LYP functional and the 6-31G and 6-31+G basis sets. A simplified achiral version of nitrone 1 as the dipole, and methyl propiolate or acrylonitrile as the dipolarophiles, were chosen as computational models. The cycloadditions have been shown to take place through one-step pathways in which the C-C and C-O sigma bonds are formed in a nonsynchronous way. For the reaction with methyl propiolate, DFT calculations predict the experimentally observed meta regioselectivity. For the reaction with acrylonitrile, however, the predicted regioselectivity has been found to depend on the computational level used. The calculations further indicate the exo approach to be energetically favored in the case of the latter dipolarophile, in agreement with experimental findings. The main reason for this is the steric repulsion between the nitrile function and one of the methyl groups on the nitrone that progressively develops in the alternative endo approach.     

Aluminium-doped catalysts for the high temperature shift reaction

The replacement of chromium by aluminium in high temperature shift catalysts was investigated. It was found that aluminium-doped hematite is a convenient candidate as an environmental friendly catalyst.     

Determination of sulfate in natural and residual waters by turbidimetric flow-injection analysis

A turbidimetric flow-injection system was developed for the determination of sulfate in natural and residual water samples, with no previous treatment, using spectrophotometric detection. The precipitating agent, 7.0% (w/v) barium chloride solution prepared in 0.10% (w/v) polyvinyl alcohol, was added by using the merging-zones approach. A 100 mg/L sulfate solution in 0.07M nitric acid was mixed with the sample before it entered the injection loop to improve the detection limit, provide in-line pH adjustment, and prevent the interference of some anionic species. The relative standard deviations of the results were between 1.4 and 3.0% and were in agreement with results obtained by the reference method. Samples within a linear concentration range of 10-120 mg SO4(2-)/L can be analyzed at a rate of 40/h. The detection limit is 5 mg SO4(2-)/L.     

On the initial treatment of kinetic data

The kinetic methods usually employed to determine reaction orders involve some sort of mathematical approximation and provide values approximate and very often discrepant. Three methods are reported to determine accurate reaction orders without introducing approximations.

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Toward an understanding of the selectivity in domino reactions. A DFT study of the reaction between acetylenedicarboxylic acid and 1, 3-Bis(2-furyl)propane

The mechanism of the domino reaction between acetylenedicarboxylic acid and 1,3-bis(2-furyl)propane has been theoretically studied in the framework of density functional theory. This domino process comprises two consecutive cycloaddition reactions: the first one is initialized by the nucleophilic attack of the C5 position of the furan ring to a conjugate position of acetylenedicarboxylic acid to give a zwitterionic intermediate, which by a subsequent ring-closure process affords an oxanorbornadiene intermediate. The second reaction is an intramolecular concerted cycloaddition of this intermediate to give the final dioxapentacyclic adduct. For the second cycloaddition, which corresponds to the step controlling the selectivity, eight alternative reaction pathways are found. Chemoselectivity, facial selectivity, and stereoselectivity of this domino reaction are related with the different approach modes of the tethered furan to the oxanorbornadiene system of the intermediate. The most favorable pathway takes place along an endo/syn approach of the furan ring relative to the bridged oxygen atom of the oxanorbornadiene system, with participation of the substituted double bond. An analysis of energetic contributions to the potential energy barriers for the intramolecular cycloadditions identifies the different factors controlling the reactive channels. Selectivity outcome is reproduced by these calculations.     

Understanding the mechanisms of [3+2] cycloaddition reactions. The pseudoradical versus the zwitterionic mechanism

Analysis of 12 three-atom-components (TACs) participating in [3+2] cycloaddition (32CA) reactions towards ethylene and acetylene allows establishing good correlations between the pseudodiradical character, the hardness η, and the nucleophilicity N index of the TAC with the feasibility of these non-polar reaction. These results allow the introduction of the pr index, which comprises the two aforementioned DFT reactivity indices. The increase of the pr index for an allylic-type TAC goes accompanied by a linear decrease of the activation enthalpy of the reaction. The present study makes it possible to establish a useful classification of 32CA reactions into zw-type reactions involving TACs with a high zwitterionic character, and pr-type reactions involving TACs with a high pseudodiradical character.     

Effect of concentration, particle size and the presence of protective coatings in DRIFT spectra of building materials

This paper discusses the effect of particle size (from under 45 to 425 μm), sample concentration (5 and 50% dilution in KBr) and the presence/absence of anti-graffiti coatings on the quality of diffuse reflectance spectra, specifically the spectra for limestone, granite and brick. In limestone, sample dilution was found to affect spectral resolution significantly, whereas the increase in particle size leads to a slight decrease in signal intensity. The presence of anti-graffiti protection was the factor that disturbed spectral quality most visibly, except in very dilute (5%) samples with a very fine particle size (under 45 μm). In more heterogeneous materials such as brick and granite, particle size proved to have a greater impact than dilution, while the presence of protective treatment was again the parameter with the greatest effect on quality and consequently signal reproducibility. This effect was slighter in very dilute samples with a small particle size.     

An MEDT study of the mechanism and selectivities of the [3+2] cycloaddition reaction of tomentosin with benzonitrile oxide

The [3+2] cycloaddition (32CA) reaction of tomentosin with benzonitrile oxide yielding a spiro-isoxazoline has been studied within the Molecular Electron Density Theory at the B3LYP/6-31(d,p) computational level. Given the multifunctionality of tomentosin, this 32CA reaction can take place along 16 competitive reaction paths. The chemo-, regio-, and stereoisomeric reaction paths involving the two C C double bonds of tomentosin have been studied. Density functional theory (DFT) calculations account for the total chemo- and regioselectivity, in complete agreement with the experimental outcomes, being suggestive of low diastereofacial selectivity. Analysis of the conceptual DFT indices accounts for the nonpolar character of this 32CA reaction. On the other hand, the topological analysis of the electron localization function of the selected points of the intrinsic reaction coordinate associated with the formation of the C C and C O single bonds emphasizes the zw-type reactivity of the phenyl nitrile oxide; the reaction taking place through a non-concerted two-stage one-step mechanism initialized with the formation of the C C single bond involving the β-conjugated carbon of tomentosin.     

On the linear complexity of the Naor–Reingold sequence with elliptic curves

The Naor–Reingold sequences with elliptic curves are used in cryptography due to their nice construction and good theoretical properties. Here we provide a new bound on the linear complexity of these sequences. Our result improves the previous one obtained by I.E. Shparlinski and J.H. Silverman and holds in more cases.